Capacitive pressure sensors are well known. They typically include a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor. A deformable conductive member, such as a metal foil diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that the central portion is substantially parallel to and opposite the fixed plate. Since the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance of the sensor changes in relation to changes in the gap g between the diaphragm and the conductive surface of the fixed element. Reducing the gap increases the capacitance, albeit not necessarily with a linear relationship. A pressure differential applied across the diaphragm causes the diaphragm to deflect and the gap to change as a function of the pressure differential. For an edge mounted diaphragm, the deformation typically approximates a parabola. U.S. Pat. No. 5,442,962 to S. Y. Lee and assigned to Setra Systems, Inc. is illustrative of this general approach.
Capacitive pressure transducers are also known for measuring the pressure of a vacuum, e.g., the vacuum generated in the process chamber of semiconductor manufacturing equipment. Vacuum measurement is often absolute--one side of the diaphragm is evacuated to a hard, near absolute vacuum (a typical value being 1.times.10.sup.-9 Torr).
Such devices, however, are highly susceptible to error arising from 1) variations in the ambient barometric pressure acting on the exterior of the transducer, 2) other external mechanical stresses such as torques or moments produced when the transducer is connected to a system, or when printed circuit boards are attached to the transducer, 3) vibrations such as those produced by a vacuum pump equipment connected to the system, 4) mechanical shock, and 5) changes in temperature causing thermal expansion and contraction.
The changes in the capacitive gap being measured are minute, e.g. 10.sup.-9 inch, and the diaphragm is extremely sensitive to stresses which can seriously reduce the accuracy and reliability of the gap measurements. Fluctuations in barometric pressure (a maximum variation being about .+-.1.5 inch of Mercury, 38 Torr) can produce a mechanical deformation of the housing which is transmitted to the diaphragm. The "fixed" electrode can also move in response to mechanical stresses introduced by barometric changes, particularly if it is mounted, directly or indirectly, on a housing component that is exposed on one side to ambient pressure and on the other to a vacuum. Deformations of the housing in the range of 1.times.10.sup.-5 inch to 1.times.10.sup.-7 inch are typical in response to barometric pressure changes of .+-.1.5 inch of Mercury. This is significant when it is desirable to resolve variations in the capacitive gap on the order of 1.times.10.sup.-9 inch. Ideally the precision of the measurement is .+-.0.25% or less of the reading.
A straight forward solution is to make the housing structure thicker, and therefore more rigid. However, this leads to a device which is large, cumbersome, and costly to manufacture. Material costs are high because sophisticated, expensive materials such as nickel-based alloys with good corrosion resistance and low coefficients of thermal expansion are typically required. Such materials are currently four to five times the cost of typical 300 series stainless steels.
U.S. Pat. No. 5,271,277 to Pandorf, U.S. Pat. No. 5,515,711 to Hinkle, and U.S. Pat. Nos. 4,823,603 and 5,396,803 to Ferran describe prior art capacitive, edge-mounted diaphragm devices which were designed to measure vacuum, or later adapted for this application. They deal with the movement of the housing in response to barometric pressure changes by 1) supporting the "fixed" electrode on a ceramic disc separate from the backplate of the housing exposed to ambient pressure and 2) supporting the diaphragm on a ring that is in turn supported from the backplate at its outer edge (where there is less movement than at its center). Connections to the electrode are made by feedthroughs in the backplate. The ceramic plate is typically spring loaded against the backplate by a stiff annular spring member to control the spacing between the plate and the diaphragm by strongly clamping them together.
The '603 Ferran patent also deals with the problem of mechanical hysteresis during movement of the ceramic disc with respect to metal parts supporting the disc. It teaches supporting the ceramic disc at its side on a set of sapphire roller bearings. This helps the hysteresis, but it is not rugged, and it has obvious cost and assembly disadvantages--beyond the cost and assembly disadvantages of the ceramic disc itself.
It is therefore a principal object of the invention to provide a housing structure for a capacitive pressure transducer which itself provides a high level of isolation of the diaphragm and the diaphragm-to-electrode gap from stresses induced by variations in the ambient barometric pressure.
A further principal object is to provide a high degree of isolation from all mechanical stress, including temperature-induced stress.
Another principal object is to provide this isolation with a compact, rugged structure that has a low cost of manufacture as compared to currently available commercial transducers with comparable operational capabilities.